The added value of metal substrates has conventionally been increased through enhancement of the functionality and designability of the substrate using a coating material or ink that contains a resin. Technical developments in recent years have spurred the demand for higher functionality and design diversity, and for coating and printing technology with high precision.
The presence of foreign matter such as dust and dirt on the surface of a substrate prior to coating or printing of the substrate surface gives rise, in coating material and inks, to a drop in wettability and in adhesion to the substrate, which precludes achieving satisfactory coating or printing.
PTL 1 discloses a method for accelerating curing of an outer-surface coating agent, the method involving removing for instance water, dust, oils and the like adhered to the surface of a steel pipe, using a burner flame, and thereafter, coating the steel pipe with a coating agent once the temperature of the steel pipe has been brought to 50° C. to 70° C.
The surface of steel pipes is in general sufficiently coated with anti-rust oil. In order to burn anti-rust oil off, heating is required at a temperature of 400° C. or above, for a given lapse of time. This may give rise to the problem of impaired quality, as an article, due for instance to oxidation of the metal surface. Accordingly, means such as alkali degreasing are ordinarily resorted to in order to remove oils, such as anti-rust oil, that are adhered to the metal surface, while removal of oils on the metal surface by burning is generally not resorted to.
PTL 2 discloses a method in which oils or solid deposits having become adhered, during a production process, on the surface of a metal strip such as a stainless steel strip or an alloy strip, are washed using a washing liquid such as an organic solvent or an alkaline solution, followed by cleaning of the surface by a flame treatment.
Although no foreign matter is present on the surface of the metal strip immediately after such degreasing and cleaning, foreign matter such as floating dust or dirt in the storage site becomes deposited during storage over the course of several hours to several days. Such foreign matter must thus be removed when performing high-precision coating or printing.
In a plated steel sheet such as a hot-dip Zn-55% Al alloy plated steel sheet, the plating surface immediately after plating is clean, and accordingly a coating material or printing ink is in sufficient close contact with the surface. Over the course of several hours or several days after plating, however, very fine organic dirt becomes adhered to the metal surface. This is problematic in that the coating material or printing ink fails to be in sufficient close contact with the surface.
In the production process of the coated steel sheet, a coating film may in some instances adhere partially to a device for producing the coated steel sheet, when the coated steel sheet comes in contact with the device. When this deposit adheres to a coated steel sheet of a subsequent load, the deposit constitutes foreign matter of that subsequent-load coated steel sheet.